Regulation of the biological clock in Drosophila
Introduction The fruit fly Drosophila melanogaster operates on a 24-hour schedule maintained by environmental input to an internal body clock. The molecular basis of the clock is oscillations in the activation of specific genes at certain times of the day. In Drosphila, RNA and protein levels of the period and timeless genes oscillate and the proteins autoregulate their transcription.(1) The key feature of these molecular oscillations is a negative feedback loop in which the protein products of genes actually turn off production of more protein. The circadian rhythm is essential for locomotor activity, eclosion behavior, and for the rhythmic component of the male courtship song that originates in the thoracic nervous system of drosophila. Regulation of the molecular clock in Drosophila melanogaster Regulation starts with the transcription of the per gene in the nucleus of Drosophila, the mRNA then moves to the cytoplasm where it is translated in the PERIOD (PER) protein. PER protein levels reach a maximum during the middle of the night. At that time, the PER molecules enter the nucleus, where they inhibit transcription of their own gene. As the sun rises, PER molecules become susceptible to degradation. Over the course of several hours, all PER protein disappears. In the absence of PER, transcription of the per gene begins again. The regulation of the circadian clock in Drosophila has 3 parts: 1. Activation of the per gene 2. Formation of a complex with the TIM protein 3. PER and TIM degradation Molecular feedback loops in Drosophila melanogaster Activation of the per gene Transcription of the per gene is regulated by two proteins: Cycle (CYC) and CLOCK. At night the CYC/CLOCK complex is bound to the promoter, and the per gene is transcribed. Transcription is repressed when PER protein molecules interact directly with the CYC/CLOCK complex. After the sun rises and PER molecules degrade, the repression of the CYC/CLOCk complex is removed and the per gene transcription resumes. PER forms a complex with TIM Like the per gene, transcription of the timeless (tim) gene is activated by the proteins CYC and CLOCK. The product of the tim and per genes are the TIM and PER proteins. These proteins bind to one another to form a heterodimer. The formation of a complex with TIM protects PER from rapid degradation. PER/TIM complexes enter the nucleus where they interact with CYC/CLOCK complexes. This interaction represses transcription of the tim and per genes. As the sun rises, light causes rapid degradation of TIM. Without TIM as a stabilizer PER also degrades. The repression of CYC/CLOCK is released, and transcription resumes. PER and TIM degradation PER protein degradation is due to the addition of phosphate groups by a Drosophila kinase named doubletime protein. When PER proteins have formed complexes with TIM proteins they are resistant to degradation by doubletime. PER/TIM complexes enter the nucleus, interact with CYC/CLOCK, and repress the transcription of the per and tim genes. Mutations in the doubletime kinase gene result in a fruit fly with a longer circadian period of about 28 hours. (2) TIM protein is inactivated by cryptochrome. When the sun rises, light activates the cryptochrome protein by a conformation change. Activated cryptochrome interacts with TIM, causing it to degrade. Without the stabilization provided by TIM, PER proteins are susceptible to degradation by the doubletime protein in the nucleus. (3). The degradation of PER/TIM results in the release of repression, and transcription of the per and tim genes begins once again. Clock/CYC heterodimer acts as transcriptional activator (positive element) for period (per) and timeless (tim) genes. The heterodimer of PER/TIM is phosphorylated in the cytoplasm, the phosphorylated complex acts as inhibitor for its own transcription (negative element). The protein Cryptochrome (CRY) has been implicated in the light entrainment pathways of the Drosophila. See video (4) Conclusions The biological clock in Drosophila melanogaster is regulated by the PER protein dosage. An increase in PER leads to shortened circadian rhythms and a decrease leads to lengthened circadian rhythms. The regulation of the cycle depends on the formation of the TIM/PER complex. Light induces the degradation of TIM, which promotes elimination of PER. The heterodimer regulates PER and TIM transcription through a negative feedback loop. References 1. Hardin PE et al. Feedback of the Drosophila period gene product on circadian cycling of its messenger RNA levels. Nature 343 (6258): 536–40. Bibcode:1990Natur.343..536H. doi:10.1038/343536a0. PMID 2. Konopka RJ and Benzer S. "Clock mutants of Drosophila melanogaster". 1971. Proc. Natl. Acad. Sci. U.S.A. 68 (9): 2112–6. Bibcode:1971PNAS...68.2112K. doi:10.1073/pnas.68.9.2112. PMC 389363. PMID 5002428. 3. Emery P. et al. CRY, a ''Drosophila ''Clock and Light-Regulated Cryptochrome, Is a Major Contributor to Circadian Rhythm Resetting and Photosensitivity. 1998.Cell, Vol. 95, 669–679. 4. https://www.youtube.com/watch?v=zB4VBpv9rn4